Communication networks whose components communicate with one another by packet switching are being used increasingly instead of line-switching communication networks. Networks such as these are frequently also referred to as Voice-Over-IP (VoIP) networks, when the Internet Protocol (IP) is used to interchange acoustic information (speech data) in the form of data. In this case, the networks may either be used exclusively for transmission of speech data or else can transmit both speech data and other information (for example data, video) as well in a mixed form.
In line-switching communication networks, terminals (for example telephones, fax machines) are always connected to a communication node (for example to a telephone system). The entire information flow to and from the terminals in this case takes place with the involvement of the respective communication node, with control information, for example for setting up links, being interchanged between the terminal and the communication node, while the payload information (the analog speech signal of the modulated fax signal) is admittedly interchanged between the terminals that are involved in the communication link but is passed on (transmitted) by at least one communication node that is involved in the communication link.
In speech data networks, the terminals are frequently referred to as “clients”, because the terminals in these communication networks are operated in a similar manner to a computer in a data network and not only as a telephone or fax machine, but may also, for example be in the form of a multimedia PC. A multimedia PC is a computer which has suitable hardware for inputting information (for example a microphone, camera, keyboard, scanner etc.) and for outputting information (for example, a screen/display, loudspeakers, LEDs etc.). Clients may also be in the form of a miniature PC (so-called PDA—Personal Digital Assistant) or an extended mobile telephone (smart phone). The appliances mentioned are often equipped with a cordless data link and, in the case of mobile telephones are then also referred to as “WLAN cellphones” (WLAN=Wireless Local Area Network).
The clients in speech data networks generally interchange the payload information directly with one another. For this purpose, once a communication link has been set up, two clients communicating with one another each interchange the data packets with the payload information with the knowledge of the network address of their communication partner, and on the basis of these network addresses. In order to set up a link, by contrast, central network nodes (communication nodes) are also used in the known speech data networks. Network nodes such as these are known to the speech data networks which use H.323 protocol (ITU-T-H.323) as “gatekeeper” and to the speech data networks which use the SIP protocol (IETF-SIP; SIP=Session Initiation Protocol), as SIP proxy servers.
Widespread data networks, in particular, the Internet, are increasingly using communication structures which are referred to as “peer-to-peer” networks or else “end-to-end” networks. One primary field of application for communication structures such as these is to interchange files, so-called “file sharing”. In this case, a subscriber (client) to a communication structure such as this checks a number of other clients (typically computers) when looking for a specific resource (which may be a music file or a computer program), and starts to use this resource once it has been found, which generally involves the transmission (“download”) of one or more files. In this case, either appropriate communication partners (clients) are found using a database, or else the clients can be found by appropriate communication methods without any central instance.
Some of the known “peer-to-peer” networks are used for the purpose of direct communication. The “media data” that is interchanged in this case, that is to say the payload data, is not restricted to speech (speech data) or modulated information (for example fax transmissions), but also includes moving image information (video transmission, video conferences, “multimedia messaging”) and other forms of electronic communication. In communication networks such as these, which are also referred to as “directly-communicating communication networks”, this means on the one hand that communication links can be set up in the form of conventional “telephone calls”, while on the other hand it is also possible to transmit extended contents (“multimedia”). Avoiding the use of central network nodes (communication nodes) in this case results in the advantage of increased resistance to failures, although it is associated with the disadvantage that the (switching) functionality of the network nodes, as is known from line-switching telephony must be provided by the clients involved in the communication link, themselves.
In the case of the known directly-communicating communication networks, it has been found to be particularly disadvantageous when a client must have the address information (for example the network address) of a second client in order to set up a communication link to that second client. This is a problem because, in general, there is no network node (for example communication server) directly associated with the client, with which it is possible to check an address such as this. Even if the address information for the desired communication partner is known, it is possible in directly-communicating communication networks that it is not possible to reach this desired communication partner (for example if it is “busy” or if the client has failed) so that, for example it is desirable to set up a link to an alternative call destination (“alternative destination”). Since directly-communicating communication networks do not use central network nodes for storage of address information such as this, the problem of addressing still exists when setting up a connection to an alternative call destination such as this.